Analog Devices AD9273, Manual

Page: 32 / 49

AD9273
Rev. B | Page 31 of 48
ADC
The AD9273 uses a pipelined ADC architecture. The quantized
output from each stage is combined into a 12-bit result in the
digital correction logic. The pipelined architecture permits the
first stage to operate on a new input sample and the remaining
stages to operate on preceding samples. Sampling occurs on the
rising edge of the clock.
The output staging block aligns the data, corrects errors, and
passes the data to the output buffers. The data is then serialized
and aligned to the frame and output clocks.
CLOCK INPUT CONSIDERATIONS
For optimum performance, the AD9273 sample clock inputs
(CLK+ and CLK−) should be clocked with a differential signal.
This signal is typically ac-coupled into the CLK+ and CLK− pins
via a transformer or using capacitors. These pins are biased
internally and require no additional bias.
Figure 55 shows the preferred method for clocking the AD9273.
A low jitter clock source, such as the Valpey Fisher oscillator
VFAC3-BHL-50MHz, is converted from single ended to
differential using an RF transformer. The back-to-back Schottky
diodes across the secondary transformer limit clock excursions
into the AD9273 to approximately 0.8 V p-p differential. This
helps prevent the large voltage swings of the clock from feeding
through to other portions of the AD9273, and it preserves the
fast rise and fall times of the signal, which are critical to low
jitter performance.
0.1µF
0.1µF
0.1µF
0.1µF
SCHOTTKY
DIODES:
HSM2812
3.3V
50
Ω
100
Ω
CLK–
CLK+
ADC
AD9273
MINI-CIRCUITS
ADT1-1WT, 1:1Z
XFMR
VFAC3
OUT
07
03
0-
05
0
Figure 55. Transformer-Coupled Differential Clock
If a low jitter clock is available, another option is to ac-couple a
differential PECL signal to the sample clock input pins as shown
in Figure 56. The AD951x/AD952x family of clock drivers offers
excellent jitter performance.
100
Ω
0.1µF
0.1µF 0.1µF
0.1µF
240
Ω
240
Ω
50
Ω *
CLK
CLK
*
50
Ω
RESISTOR IS OPTIONAL.
CLK–
CLK+
ADC
AD9273 PECL DRIVER
3.3V
OUT
VFAC3
07
03
0-
05
1
AD951x/AD952x
FAMILY
Figure 56. Differential PECL Sample Clock
100
Ω
0.1µF
0.1µF 0.1µF
0.1µF
AD951x/AD952x
FAMILY
50
Ω *
CLK
CLK
*
50
Ω
RESISTOR IS OPTIONAL.
CLK–
CLK+
ADC
AD9273
LVDS DRIVER
3.3V
OUT
VFAC3
07
03
0-
05
2
Figure 57. Differential LVDS Sample Clock
In some applications, it is acceptable to drive the sample clock
inputs with a single-ended CMOS signal. In such applications,
CLK+ should be driven directly from a CMOS gate, and the
CLK− pin should be bypassed to ground with a 0.1 μF capacitor
in parallel with a 39 kΩ resistor (see Figure 58). Although the
CLK+ input circuit supply is AVDDx (1.8 V), this input is
designed to withstand input voltages of up to 3.3 V, making the
selection of the drive logic voltage very flexible.
0.1µF
0.1µF
0.1µF
39k
Ω
CMOS DRIVER
50
Ω *
OPTIONAL
100
Ω
0.1µF
CLK
CLK
*
50
Ω
RESISTOR IS OPTIONAL.
CLK–
CLK+
ADC
AD9273
3.3V
OUT
VFAC3
07
03
0-
05
3
AD951x/AD952x
FAMILY
Figure 58. Single-Ended 1.8 V CMOS Sample Clock
0.1µF
0.1µF
CMOS DRIVER
50
Ω
*
OPTIONAL
100
Ω
0.1µF
CLK
CLK
*
50
Ω
RESISTOR IS OPTIONAL.
CLK–
CLK+
ADC
AD9273
3.3V
OUT
VFAC3
0.1µF
07
03
0-
05
4
AD951x/AD952x
FAMILY
Figure 59. Single-Ended 3.3 V CMOS Sample Clock
Clock Duty Cycle Considerations
Typical high speed ADCs use both clock edges to generate a
variety of internal timing signals. As a result, these ADCs may
be sensitive to the clock duty cycle. Commonly, a 5% tolerance is
required on the clock duty cycle to maintain dynamic performance
characteristics. The AD9273 contains a duty cycle stabilizer (DCS)
that retimes the nonsampling edge, providing an internal clock
signal with a nominal 50% duty cycle. This allows a wide range
of clock input duty cycles without affecting the performance of
the AD9273. When the DCS is on, noise and distortion perfor-
mance are nearly flat for a wide range of duty cycles. However,
some applications may require the DCS function to be off. If so,
keep in mind that the dynamic range performance can be affected
when operated in this mode. See Table 17 for more details on
using this feature.